In the central atrium, inmates of all ages and ethnic backgrounds, dressed mostly in prison-issue grey tracksuits and trainers, were standing and sitting around in small groups on the plastic benches and tables, which she noticed were bolted to the floor.

Accompanied by a female prison officer, she was marched through the atrium and up the metal stairs to her cell, passing the inmates as they lounged on the landings and leant on the banisters, chatting and watching each other, some of them breaking off their conversations to observe her as she walked by. Most of the gazes were flat and impenetrable, others were curious, some were downright hostile. As she passed by one group of inmates, she heard one of them mutter something she couldn’t quite make out, which got some sniggers in response. She knew it had been directed at her, but she kept her head down and avoided eye contact.

She felt that familiar gut-twisting fear return, that they would somehow see through her and see her for what she actually was – a serving police officer. But despite that, she found herself falling back into her undercover role with an ease she hadn’t been expecting, especially considering what had happened on the previous job. And, what was more, on the crest of the fear, she felt that old buzz, the reason she’d started working undercover in the first place. Only now, after her extended absence from this kind of work, did she realise just how much she craved it.

As for her cellmate, Sharon, her cover story seemed to have done the trick for the time being. Sharon seemed affable enough, but Bailey didn’t trust her one inch. She had that familiar animal cunning that so many criminals seemed to possess. As a policewoman, Bailey had encountered it innumerable times. Although not necessarily intelligent as such, these people had an innate deviousness that it was well to be wary of and fatal to underestimate.

The undercover training course had emphasised that it was always best to try to keep your cover story as true to life as possible. The more you lied, the more you were at risk of holes showing in your story. And the more you tried to be what you weren’t, the more likely people were to see through you. Criminals were paranoid people by their very nature, and often on the lookout for anything suspicious. So it had been no lie about the accountancy. Bailey really had once been an accountant. For about five minutes. She’d studied it at college, thinking that it would make a good solid career. But as a job, she’d found it so boring that she’d quit after two months and applied to join the police instead, something she’d wanted to do ever since she was a kid. The upshot of this, however, was that when it came to financial records and bookkeeping, spreadsheets and payroll, she could talk with some authority and sound natural.

And that was important, because maintaining a cover story required discipline and the ability to stay on the ball all the time, however tired you were. One thing the training course had emphasised was that the effort of sustaining a facade, the pressure of having to lie, and the consequences of making mistakes could take a heavy psychological toll, especially over an extended period of time.